High performance energy storage devices are critical in advanced transportation technologies, e.g., electrical vehicles (EVs) and hybrid electrical vehicles (HEVs). This is extremely useful in achieving better fuel economy, decreasing harmful emissions, and reducing our nation's reliance on foreign sources of petroleum.
Ultracapacitors (aka, supercapacitors, Electrical Double-Layer Capacitor (EDLC), or electrochemical capacitors) are being developed as power assists for HEVs. Ultracapacitors are energy storage devices which combine the high-energy storage potential of batteries with the high-energy transfer rate and high recharging capabilities of capacitors. Ultracapacitors can have orders of magnitude more energy density than conventional capacitors and power density than batteries. Generally, ultracapacitors have energy densities in the range of about 1 to about 10 Wh/kg, which is about one-tenth of that of secondary batteries, which have energy densities of about 20 to about 100 Wh/kg and power densities in the range of about 1000 to about 2000 W/kg, which is about ten times higher than those of secondary batteries, which have power densities in the range of about 50 to about 200 W/kg.
Energy storage in ultracapacitors can be either Faradaic or non-Faradaic. A common type of Faradaic ultracapacitor is a redox ultracapacitor in which electrons are transferred across the electrode interface. Such ultracapacitors are based on mixed metal oxides, such as ruthenium dioxide and other transition metal oxides. For reasons of high cost, scarcity, and toxicity of suitable metal oxides, Faradaic ultracapacitors are generally not preferred in most applications. In non-Faradaic ultracapacitors, no electron transfer takes place across the electrode interface, and the storage of the electric charge and energy is electrostatic. Positive and negative charges accumulate electrostatically on the electrodes at the electrode-electrolyte interface. Electrical energy is stored in the electric double layer from charge separation, i.e., the electrostatic force between an ionically conducting electrolyte and a conducting electrode. The ions displaced in forming the double layers are transferred between electrodes by diffusion through the electrolyte.
In both Faradaic and non-Faradaic ultracapacitor systems, capacitance is dependent on the characteristics of the electrode material. Ideally, the electrode material should be electrically conducting and have a porous structure. The characteristics of the porous structure, including pore size, pore size distribution, and pore volume fraction, can enable the formation of a large amount of surface area that can be used either for the development of the electrical double layer for static charge storage to provide non-Faradaic capacitance or for the reversible chemical redox reaction sites to provide Faradaic capacitance.
A major obstacle to the applications of ultracapacitors to advanced HEVs is the limited performance (energy and power densities), unsafe operation, and short cycle life of ultracapacitors. These parameters are strongly determined by the poor properties of the currently employed electrode materials (e.g., low electrolyte accessibility and low capacitance) and electrolytes (e.g., narrow electrochemical window, flammability, toxicity, volatility, thermal instability, and electrolyte depletion).
There is thus a need for an ultracapacitor capable of providing high energy and power densities, safe operation, and long cycle lives. This will need improved properties for both the electrode and electrolyte materials over those employed in current ultracapacitor technology.